Some users report munin is broken because munin and munin-node disagree about the name of the machine. I think this occurs if hostname (used by munin-node) reports a different name than PRIMARY_HOSTNAME (which we put in the munin config).

Hard-code PRIMARY_HOSTNAME in munin-node.conf.

Fixes #474.
See https://discourse.mailinabox.email/t/404-not-found-on-admin-munin/623/24.